Method of manufacturing thin-film solar cells

ABSTRACT

A method of manufacturing thin-film solar cells that include a layer of copper indium selenide (CuInSe 2 ) that is applied in one manufacturing step onto a substrate that includes a metal layer that defines an electrical back contact layer of the solar cell. A layer that contains an alkali metal, such as sodium, is formed on the cell structure before the CuInSe 2  layer is applied.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing thin-filmsolar cells.

2. Description of the Related Art

CuInSe₂ (copper indium selenide)--thin-film solar cells are normallyproduced by depositing a molybdenum (Mo) back contact onto a substrate,for instance a glass substrate. Copper, indium and selenide are thendeposited onto the back contact to form a CuInSe₂ layer, the CuInSe₂being formed by chemical reaction in a selenium-containing atmosphere.In this stage of manufacture, the structure is subjected to atemperature in the range of 350-600° C.

The CuInSe₂ layer is a p-type semiconductor. An n-type semiconductor isdeposited on the CuInSe₂ layer to form a pn-junction, which forms theactual solar cell. The n-type semiconductor is most often comprised of athin layer of CdS and a much thicker layer of ZnO, which provides thebest structure. The ZnO layer forms a front side contact at the sametime.

It is desired to increase the efficiency of solar cells of this kind.

The present invention provides a method of manufacturing solar cellswhich greatly increases the efficiency of the solar cell produced.

SUMMARY OF THE INVENTION

Accordingly, the present invention relates to a method of manufacturingthin-film solar cells wherein a structure which includes a metal layerthat forms an electrical back contact in the solar cell is coated with alayer of copper indium selenide (CuInSe₂) in a manufacturing stage. Theback contact is applied to a substrate, wherein the method ischaracterized by applying a layer of alkali metal to the structure priorto applying the CuInSe₂ layer.

According to one particularly preferred embodiment of the invention, thealkali metal that is applied to the structure prior to applying theCuInSe₂ layer is sodium (Na) or potassium (K).

By copper indium selenide layer as used in the present context and inthe claims, is meant a copper indium selenide layer of differentcompositions and containing different alloying substances, primarilygallium and sulphur. The term copper indium selenide as used hereconcerns primarily the compounds CuInSe₂, CuInxGa_(1-x) Se₂ andCuInxGa_(1-x) SySe_(2-y).

The invention will be described below with reference to a layer ofcopper indium selenide that has the formula CuInSe₂.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail partly withreference to an exemplifying embodiment of the invention shown in theaccompanying drawing, FIG. 1 illustrates a solar cell structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is based on the understanding that when sodium orelementary potassium in the form of elementary sodium or potassium, or acompound in which these basic substances are present, is applied to thesurface on which the CuInSe₂ layer is to be built-up, for instance tothe molybdenum layer, the grains in the polycrystalline CuInSe₂ filmwill be orientated more in a columnar structure. The grains will also belarger and the structure more dense. The resistivity in the CuInSe₂layer is also reduced, meaning that a more effective p-doping isobtained, therewith resulting in a higher cell voltage.

Alkali metals other than sodium and potassium will provide the sameeffect. This applies at least to lithium, which has a low atomic weight.

FIG. 1 is a schematic sectional view of a thin-film solar cell. Thesubstrate on which thin-film solar cells are built up may be large insize, for instance have a size of 1×0.4 meter. The substrate surfacewill therewith carry a very large number of cells that connect togetherelectrically. FIG. 1 shows only one part of one such cell. The techniqueof producing a large number of separate but electrically connected cellson a substrate surface is well known to the art and will not bedescribed in detail here.

A solar cell to which the invention can be applied nay be constructed inthe following way. The substrate 1 will normally have the form of aglass sheet of suitable thickness, for instance a thickness of 2.0 mm. Amolybdenum (Mo) layer 2 is first sputtered onto the glass surface. Themolybdenum layer forms the back contact layer and the positive terminalof the finished cell. The molybdenum layer may have a thickness of 1,000nanometers. There is then applied a CuInSe₂ layer 3, which may be givena thickness of 2,500 nanometers for instance. There is first applied tothe layer 3 a cadmium sulphide (CdS) layer 4, which may be given athickness of 50 nanometers for instance, and thereafter an electricalcontact layer in the form of a transparent doped zinc-oxide layer (ZnO)5, which may be given a thickness of 500 nanometers, for instance.

When sunlight falls on the solar cell, an electric voltage is producedbetween the electrical contact layer 5 (ZnO), which is the negativeterminal, and the electrical back contact 2.

The invention is described below with reference to the use of sodium byway of example. It will be understood, however, that other alkali metalsmay be used with equal effect. It will also be understood that althoughthe layer 6 of the illustrated solar cell is referred to as sodium, thelayer 6 will in fact consist of the alkali metal that is used when thisalkali metal is other than sodium.

According to the invention, a layer 6 containing an alkali metal, in theillustrated case sodium (Na), is formed on top of the back contact layer2, i.e. on top of the molybdenum layer of the illustrated example, priorto applying the CuInSe₂ layer 3.

According to one preferred embodiment of the invention, sodium isapplied on the electrical back contact layer, i.e. the metal layer 2 byvaporizing sodium selenide (Na₂ Se).

According to another preferred embodiment of the invention, potassium isapplied on the electrical back contact layer, i.e. the metal layer 2 byvaporizing potassium selenide (K₂ Se).

According to another preferred embodiment, the alkali metal, in theillustrated case sodium, is applied simultaneously with the electricalback contact layer when said back contact layer is comprised ofmolybdenum.

According to another preferred embodiment of the invention, the layerwhich includes the alkali metal, in the illustrated case sodium, isapplied to a thickness of from 50 to 500 nanometers.

According to another embodiment, the alkali metal layer also containsoxygen (O).

It has been found that when the CuInSe₂ layer is applied on top of thelayer that contains sodium or potassium, the sodium or potassium willessentially disappear from the surface of the metal contact layer. Whenthe CuInSe₂ layer is applied, sodium or potassium is again found at thegrain boundaries in the CuInSe₂ layer and on the surface of the layer.When the CdS layer is then applied in a wet process, the sodium or thepotassium compound will disappear from the surface, provided that thesodium or the potassium compound is soluble in the liquid used in thewet process. An electric contact layer is applied on top of the layer ofcadmium sulfide, and that electric contact layer is preferably a layerof doped zinc oxide.

This method results in a very surprising increase in the efficiency ofthe solar cell described and shown by way of example, namely an increaseof about 25%. The typical efficiency of such a solar cell of this natureto which the invention has not been applied is 12%. The efficiencyincreases to 15% when the invention is applied.

Although the present invention has been described with reference to agiven solar cell structure, it will be understood that the presentinvention can be applied to other structures in which the electricalback contact layer 2 is not comprised of molybdenum but of some othersuitable metal, such as tungsten, nickel, titanium or chromium.

Furthermore, substrates other than a glass that contains sodium may alsobe used. For instance, a glass which does not contain sodium may beused, as may also glass which contains sodium and which includes adiffusion barrier against sodium on the surface on which the electricalback contact layer is to be built up.

The invention shall not therefore be considered to be limited to theaforedescribed exemplifying embodiments thereof, since variations can bemade within the scope of the following claims.

What is claimed is:
 1. A method of manufacturing thin-film solar cells,said method comprising:a. providing a substrate; b. applying a metalliclayer to a surface of the substrate, the metallic layer defining anelectrical back contact layer; c. depositing on the electrical backcontact layer an intermediate layer containing an alkali metal; d.forming on the intermediate layer a layer containing copper indiumselenide to provide a p-type semiconductor layer; and e. depositing onthe p-type semiconductor layer a layer of n-type semiconductor materialto provide a solar cell having increased efficiency.
 2. A methodaccording to claim 1, wherein the intermediate layer contains the alkalimetal sodium (Na).
 3. A method according to claim 2, wherein said sodiumis applied on the electrical back contact layer by vaporizing sodiumselenide (Na₂ Se).
 4. A method according to claim 1, wherein the alkalimetal-containing layer is applied simultaneously with the electricalback contact layer by sputtering and said electrical back contact layerincludes molybdenum.
 5. A method according to claim 1, wherein the layercontaining the alkali metal has a thickness of from about 50 to about500 nanometers.
 6. A method according to claim 1, wherein the layerwhich includes the alkali metal also includes oxygen (O).
 7. A methodaccording to claim 1, wherein the electrical back contact layer ismolybdenum (Mo).
 8. A method according to claim 1, wherein the n-typesemiconductor layer is formed by applying cadmium sulphide (CdS) on topof the copper-indium-selenide-containing layer, and by applying a secondelectrical contact layer on top of the layer of cadmium sulphide.
 9. Amethod according to claim 8, wherein the second electrical contact layeris a layer of doped zinc oxide (ZnO).
 10. A method according to claim 1,wherein the intermediate layer contains the alkali metal potassium (K).11. A method according to claim 10 wherein said potassium is applied onthe electrical back contact layer by vaporizing potassium selenide (K₂Se).